Radioactive solid particles for use in well logging



3,070,696 RADIOACTIVE SOLID PARTICLES FOR USE IN WELL LOGGING Cassius R.McEwen, Fullerton, Calif, assignor to Union Oil Company of California,Los Angeles, Calif., a corporation of California No Drawing. Filed Sept.19, 1955, Ser. No. 535,291

4 Claims. (Cl. 25083.3)

This invention relates to radioactive solid particles suitable for usein certain well logging operations, and to a method for preparing thesame. It further relates to a well logging method in which suchparticles are obtained.

The use of radioactive substances in determining the variations in thepermeabilities of subterranean strata traversed by a well bore is wellknown. According to one embodiment of such methods, a fluid such ascrude oil or brine carrying suspended radioactive solid particles ispumped down the borehole under a pressure sufiicient to force the fluidout into the strata traversed by the bore. The various strata act asfilters to remove the suspended solids from the suspending liquid. Sincethe quantity of fluid which is forced into any one stratum is directlyproportional to the permeability of that stratumwith respect to thesuspending liquid, the quantity of radioactive solids which is filteredout and depositedas a filter cake on the wall of the borehole oppositesuch stratum is like; wise directly proportional to such permeability.Accord ingly, after a brine suspension of radioactive solid particleshas been forced down the bore under the aforesaid pressure the walls ofthe bore will bear a coatingof radioactive solids, and the thickness ofsuch coating at any point will be a measure of the permeability withre-.

spect to the brine of the particular stratum which underlies the coatingat that point. Since the intensity of radiation from said coating isdirectly proportional .to the thickness of the coating, a log of thethickness of the coating (and hence of the brine permeabilities of thestrata which underlie the coatinglcan readily be obtained by passing aradiation detector, e.g., a Geiger-Muller counter, through the bore andplotting the radiation intensity versus depth. When it is desired todetermined the per.- meabilities of the strata with respect to a fluidother than brine, such fluid is employed for suspending the radioactiveparticles.

As will readily be appreciated, the success and accuracy of the loggingmethod described above will depend upon the radioactive solid particlespossessing certain requisite characteristics. light in weight, ideallyhaving the same density-as the suspending liquid, so that they mayreadily be maintained uniformly suspended in the suspending liquid.Also, they must be inert with respect to the suspending liquid and theenvironment in which it is employed; that is, the radioactive element inthe solid particles should be of such nature and should be so combinedwiththe particles that 'it is not leached out by the suspending liquidor replaced by ions or other entities contained in such liquid, and isnotabsorbed by or in the subterranean-strata, Furthermore, the particlesshould be of such nature that they are readily filterable, i.e., theyshould not swell or otherwise change physical form so as to clog thestrata and impede the filtering action. At the present time theradioactive solid most widely employed is obtained by treating ananion-exchange resin with a radioactive iodide and thereafter treatingthe exchanged resin with silver nitrate to precipitate radioactivesilver iodide on the resin particles. Such material, however, is notsatisfactory for use where the suspending liquid is an oil field brinecontaining sulfide ion since the latter chemically replaces 3,070,696Patented Dec. 25, 1962 the iodine in the silver iodide and theradioactive iodine is carried into the strata by the brine.

I have now found that a highly superior radioactive solid suitable foruse in logging well bores by the method described above can be preparedby treating a solid polyclefine or a solid vinyl polymer with aradioactive halo- 0d by which the new products are prepared is verysimple and inexpensive, and requires no special equipment or procedure.If necessary, they can be prepared at the Well site.

The polymeric solid which is treated with a radioactive halogen inaccordance with the invention is preferably solid polyethylene, e.g.,ordinary commercial polyethylene such as is widely employed forelectrical insulating purposes and for the manufacture of flexiblecontainers such as the familiar squeeze bottle. Alternatively,

other polyolefines such as polypropylene andvinyl polyfunctional groupsare excluded. The term non-ion exchanging polyvinyl resin is hereinemployed to designate the present class of vinyl polymers and todistinguish over the prior art use of ion-exchanging polyvinyl resinssuch as the sulfonated polystyrenes.

Since the logging method to which the invention per tains operates onthe principle of the radioactive solids being filtered out of thesuspending liquid and forming a filter cake of varying thickness on thewalls of the bore, the size of such particles will depend upon thenature of the subterranean strata traversed by the bore,'i.e., such sizeshould be larger than the interstices or pores of the strata to belogged in orderto avoid the particlesbein-g carried into suchinterstices or pores. In general,'such particle size will correspond tobetween about 30- and Thus, they must be relatively about 300 -mesh,although particles of larger or smaller sizes maybe employed where thestrata have unusually large or unusually small pores. In order to attaingood filtering action and to avoid clogging the pores of the strata itis desirable to employ as large a particle size as is consistent withaccurate results and the maintenance of .a relatively uniform andvstable suspension in the suspending liquid. q

i As previously explained, the radioactive element with I which thepolyethylene or polyvinyl resins particles are treated in accordancewith the invention is a radioactive halogen, e. g., chlorine 36, bromine82, iodine 126, iodine 130, iodine 131, etc. By reason of the ease andrapidity with which the products of the invention are prepared it ispossible to employ radioactive halogens havinghalflives as short asseveral hours. However, radioactive isotopes of iodine, e.g., iodine131, are preferred by reason of their ready availability. It should benoted that in the practice of the present invention the radioactiveelement is employed in elemental form rather than as a salt or othercompound thereof.

The step of treating the polyethylene or polyvinyl resin particles withthe radioactive halogen consists simply in contacting the solidparticles with the halogen at ambient or slightly elevated temperaturesuntil sufiicient of the halogen has combined with the resin to give asolid having the desired radiation intensity. The latter will begoverned by the sensitivity of the radiation detector employed inrunning the log and the conditions under which the log is run.Ordinarily, the contacting time is between about 2 and about 20 hours.The radioactive halogen is most conveniently employed in the form of asolution in a solvent which is a non-solvent for the resin. Solventssuch as water and parafiinic hydrocarbons are preferred, and theconcentration of the halogen solution may vary over wide limitsdepending upon the radiation intensity of the halogen. Suitablesolutions may contain from as low as 0.01 to as high as 100millicuries/milliliter.

Upon completion of the contacting step the treated resin particles arewashed, suitably with water or the pure solvent, to remove the treatingsolution and are dried. Depending upon the half-life of the radioactiveelement employed they may be stored for varying periods of time prior touse. Conventional techniques are employed in preparing suspensions ofthe radioactive particles for use in the logging operation, i.e., theusual suspending fluids, e.g., 'oil or brine, may be used (provided, ofcourse, the particular resin which the particles comprise is not solubletherein), suspending agents such as bentonite or organic dispersingagents and weighting agents may be provided, and the suspension maycontain the usual concentration of the radioactive particles.

The following example will illustrate one way in which the principle ofthe invention has been applied, but is not to be construed as limitingthe same:

Example I Approximately parts by weight of comminuted flexible solidpolyethylene, having an average particle size of about 30-mesh, wereplaced in a vessel containing about 100 parts by weight of solution ofradioactive iodine in iso-octane. The iodine was employed in the form ofmixed radioactive isotopes in admixture with an inert carrier materialin a ratio of about 1000 parts of carrier material per part of iodine,and the solution was prepared by dispersing about 1 part of theiodine-carrier combination in 100 parts of iso-octane. The solutioncontained about 0.01 millicurie of radioactive iodine per milliliter.The resin particles were allowed to stand in the iodine solution forabout hours, after which the solution was filtered off and the treatedresin particles were washed with hexane and dried.

When the resulting product was continuously leached with a typical oilfield brine containing soluble sulfides it was found that it required 9liters of brine to reduce the radioactivity of the product by-50%. In asimilar test conducted with a product prepared by precipitatingradioactive silver iodide in an ion-exchange resin it was found thatonly 0.7 liter of brine was required to reduce the radioactivity by 50%.

Other modes of applying the principle of my invention may be employedinstead of those explained, change being made as regards the materialsor methods employed, provided the steps or the products stated by thefollowing claims, or the equivalent of such stated steps or products, beemployed or explained.

I, therefore, particularly point out and distinctly claim as myinvention:

1. The method of logging boreholes which comprises in a finely-dividedsynthetic resin selected from the class consisting of polyethylene andnon-ion-exchanging polyvinyl resins, said synthetic resin havingpreviously been rendered radioactive by treatment with a radioactivehalogen in elemental form, applying suflicient pressure to force saidfluid'into the strata traversed by the bore, whereby said solid aredeposited on the walls of the bore, and thereafter determiningvariations in the intensity of radiation from said deposited solids withrespect to the depth thereof.

2. The method of logging boreholes which comprises introducing into thebore a fluid having suspended therein 7 finely-divided radioactivesolids under sufficient pressure to force said fluid into the stratatraversed by the bore, whereby said solids are deposited on the walls ofthe bore, and thereafter determining variations in the intensity ofradiation from said deposited solids with respect to the depth thereofsaid radioactive solids being those prepared by allowing a comminutedresin selected from the class consisting of polyethylene andnon-ion-exchanging polyvinyl resins having an average particle sizebetween about 30 and about 300 mesh to remain in contact with a solutionof an elemental radioactive halogen in a liquid which is a non-solventfor said resin for g3. period of about 2 hours at ambient temperature inthe absence of a halogenation catalyst followed by washing said solutionfrom the resin so treated and drying the washed resin. 0

3. The method of logging boreholes which comprises introducing into thebore a fluid having suspended therein finely-divided radioactive solidsunder suflicient pressure to force said fluid into the strata traversedby the bore, whereby said solids are deposited on the walls of the bore,and thereafter determining variations in the intensity of radiation fromsaid deposited solids with re spect to the depth thereof saidradioactive solids being those prepared by allowing a comminuted resinselected from the class consisting of polyethylenue andnon-ionexchanging polyvinyl resins having an average particle sizebetween about 30 and about 300 mesh to remain in contact with a solutionof a radioactive isotope of iodine in a liquid which is a non-solventfor said resin for a period of at least about 2 hours at ambienttemperature in the absence of a halogenation catalyst followed bywashing said solution from the resin so treated and drying the washedresin. 7

4. A method as defined by claim 3 wherein the said resin employed inpreparing said radioactive solids is polyethylene.

References Cited in the file of this patent UNITED STATES PATENTS GreatBritain Nov. 22, 1950

1. THE METHOD OF LOGGING BOREHOLES WHICH COMPRISES INTRODUCING INTO THEBORE A FLUID HAVING SUSPENDED THEREIN A FINELY-DIVIDED SYNTHETIC RESINSELECTED FROM THE CLASS CONSISTING OF POLYETHYLENE ANDNON-ION-EXCHANGING POLYVINYL RESINS, SAID SYNTHETIC RESIN HAVINGPREVIOUSLY BEEN RENDERED RADIOACTIVE BY TREATMENT WITH A RADIOACTIVEHALOGEN IN ELEMENTAL FORM, APPLYING SUFFICIENT PRESSURE TO FORCE SAIDFLUID INTO THE STRATA TRAVERSED BY THE BORE, WHEREBY SAID SOLIDS AREDEPOSITED ON THE WALLS OF THE BORE, AND THEREAFTER DETERMININGVARIATIONS IN THE INTENSITY OF RADIATION FROM SAID DEPOSITED SOLIDS WITHRESPECT TO THE DEPTH THEREOF.